DTMF signaling is used in telecommunications as a form of signaling over analogue and digital telephone lines in the voice-frequency band between telephone handsets and other communication devices, as well as between communication devices without human involvement DTMF signaling and the protocols based on the DTMF signaling were designed to work well in circuit-switched networks, where both the voice and the DTMF share the same frequency band but cannot go through at the same time. Thus, the DTMF signaling in circuit-switched networks is said to be carried in-band. The sending endpoint generates DTMF tones. The receiving endpoint, when required, listens for the DTMF tones by deploying a device called a DTMF detector, a device that detects DTMF tones and reports them to call control.
To guard against false signal detection, for example voice detected as a DTMF tone, DTMF detectors have to be configured not to recognize DTMF signals whose duration is below a certain minimum. To guard against erroneous double digit detection, if a signal is interrupted by a short break in transmission or by a noise pulse and once the DTMF digit detection has started, interruptions shorter than a specified minimum must not be recognized by DTMF detectors. As an example of double digit detection, when a sending endpoint sends DTMF signals “123456789”, the DTMF detector at the receiving endpoint could detect and report “11234556678899”.
If the DTMF has to go through a packet-switched network, it can be carried either in-band or out-of-band. When DTMF signaling is carried in-band through a packet-switched network, the DTMF is treated as voice and the DTMF signaling goes through the packet network undetected. There are several issues with carrying DTMF signaling in-band through packet-switched networks. First, only some voice codecs, for example G711, can encode the DTMF signal accurately. Most compression algorithms would change the signal in such a way that it cannot be detected reliably after decoding. This means that packet-switched networks would not be able to take advantage of voice compression when DTMF signaling is required in a call. Second, packet jitter, packet delay, and/or packet loss, all of which are inherently present in packet-switched networks, can cause breaks in DTMF signals that are longer than the accepted minimum. As a result, DTMF detectors could interpret such DTMF signals either as double digits or digits can go undetected all together.
To avoid those issues described above, a more reliable method for carrying DTMF through packet-switched networks is devised whereby DTMF signals are detected via DTMF detectors at the ingress of the packet-switched network and then sent as special DTMF signaling packets into the packet-switched network, either as a substitute for the in-band DTMF, or concurrently with the packetized in-band DTMF, and thus the name out-of-band DTMF. One example of a packet-switched network is an IP network and an example of a protocol used to transport voice through an IP network is RTP, specified in IETF documents RFC3550/RFC3551, and RFC4733, that describes how to carry DTMF signaling, other tone signals and telephony events in RTP packets, that is, out-of-band.
The process of detection of DTMF signals takes a finite amount of time. Once a DTMF signal is detected, the DTMF detector reports this event to call control. It takes a certain amount of time for this to be processed by call control and for out-of-band DTMF signaling packets to begin to be injected. During this time the in-band DTMF continues to be carried through and represents in-band DTMF leak. Thus, even if the out-of-band DTMF signaling is used exclusively through the packet switched network, i.e., the packet network is configured to carry only the out-of-band DTMF signaling and to drop the in-band DTMF, some amount of the in-band DTMF signals may end up been sent as a packetized voice/audio before the out-of-band packets start being sent because it takes a finite amount of time to reliably detect a DTMF digit.
If a call carrying out-of-band DTMF signaling is terminated within the packet-switching network, the receiving endpoint within this network consumes the special DTMF signaling packets, for example RFC4733 RTP packets, and the DTMF signaling stays in out-of-band form. The leaked through in-band DTMF does not impact the ability of the receiving endpoint to recognize and interpret the out-of-band DTMF signaling packets and to act upon them. Even though the leaked in-band DTMF could be heard at the receiving endpoint, this does not impact the signaling decisions of the receiving endpoint because the receiving endpoint acts upon the out-of-band DTMF signals rather than upon the in-band DTMF signals.
If, on the other hand, the packet-switching network is just an intermediate network and the call has to be routed back into a circuit-switched network to reach its receiving endpoint, the out-of-band DTMF signaling has to be converted back to in-band DTMF form at the egress of the packet-switching network before it can be inserted into the circuit-switched network. Now a mix of the leaked in-band DTMF and the regenerated in-band DTMF is used. Depending on the amount of the leaked in-band DTMF, its relative position and phase to the regenerated in-band DTMF, and characteristics of the downstream DTMF detector, either the one at the receiving far endpoint or another intermediate one, the DTMF detector can interpret this as a double digit.
A need therefore exists for a system preventing double digit detection caused by in-band DTMF signaling and methods thereof that overcome those issues described above. These, as well as other related advantages, will be described in the present disclosure.